In an effort to devise novel therapies for diseases, injuries, and birth defects that affect the craniofacial skeleton, more needs to be done to understand how mesenchymal cells differentiate into osteocytes and make bone. To address this issue, we manipulate in vivo a highly accessible embryonic population, the cranial neural crest mesenchyme (NCM), which produces all of the bones in the facial and jaw skeletons. In published work from our prior R01 award, and in preliminary studies, we observe that NCM autonomously synchronizes and directs osteogenic induction, proliferation, differentiation, matrix deposition, mineralization, and matrix remodeling. How NCM accomplishes such a complex task, and what factors are sufficient to replicate this phenomenon, is unknown. Likely candidates may include members and targets of the Transforming Growth Factor-Beta (TGF) and Bone Morphogenetic Protein (BMP) pathways such as Runx2, Dlx5, and Msx1, since they are known to affect osteogenesis and their expression is altered in chimeras. Yet BMP4 treatments without adjacent tissues, or Runx2 over-expression alone, cannot produce premature bone, implying that combinations of signals are needed. Therefore, we hypothesize that NCM elicits positive and negative regulation by the TGF and BMP pathways to govern the timing and sequence of osteogenic events. To test our hypothesis, we exploit the divergent developmental programs of quail and duck. We transplant faster- maturing quail donor NCM into a slower-developing duck host, which creates chimeric quck; and we transplant slower duck donor NCM into the relatively faster quail host, generating chimeric duail. This provides a unique way to manipulate signaling between NCM and adjacent host tissues, and allows discovery of NCM-dependent processes. Also, all quail cells can be detected via a ubiquitous nuclear marker not present in duck. We propose three complementary and non-interdependent Specific Aims. Specific Aim 1 will determine the extent to which NCM uses TGF and BMP signaling to control osteogenic induction, proliferation, and differentiation. Specific Aim 2 will determine the extent to which NCM relies on TGF signaling to direct the timing of mineralization. Specific Aim 3 will determine the extent to which NCM enlists targets of TGF signaling including RANKL and OPG to spatiotemporally regulate osteoclasts, matrix remodeling, and bone growth. We employ gain- and loss-of-function techniques to identify molecular mechanisms that endow NCM with the ability to exert temporal control over osteogenesis. Each Specific Aim has particular clinical relevance and can serve as a proof-of-principle that molecular-based therapies can be devised to treat disorders that affect the timing of osteogenesis. Moreover, identifying mechanisms through which donor NCM transduces its effects on host cells such as osteoclasts has implications for repair and regeneration of bones injured by trauma or diseases like osteoporosis and osteonecrosis. We are hopeful that our research will provide a foundation for biologically based, non-surgical methods to remedy a variety of clinical skeletal conditions.